Optical coherence tomography (OCT) is an interferometric imaging technique with widespread applications in ophthalmology, cardiology, gastenterology and other fields of medicine. Huang D, Swanson E A, Lin C P, Schuman J S, Stinson W G, Chang W, Hee M R, Flotte T, Gregory K, Puliafito C A, and Fujimoto J G, “Optical coherence tomography,” Science, Vol 254, 1178-1181 (1991). The ability to view subsurface structures with high resolution (2-15 μm) through small-diameter fiber-optic probes makes OCT especially useful for minimally invasive imaging of internal tissues and organs.
Time-domain OCT systems employ a broadband light source as an input to an interferometer with a mechanically actuated reference arm for path-length scanning. The interference signals generated by reflections from structures at different depths are measured point-by-point as the reference path length changes. In this measurement scheme, the maximum scanning speed is limited both by the dynamic mechanical constraints of the actuator and by the spectral power density of the light source. In such a system using a superluminescent light source that emits an output power of 25 mW over a spectral bandwidth of 40-60 nm, the maximum depth-scanning velocity that can be achieved while maintaining an adequate signal-to-noise ratio for tissue imaging (>90 dB) is approximately 25 m/s. Therefore, 512-line images of a 5 mm deep object can be acquired at a rate no greater than 10 per second.
Frequency-domain (also called Fourier-domain) (FD) OCT overcomes these speed constraints by taking advantage of optical frequency discrimination methods based on Fourier transformation, which eliminate the need for long-range mechanical actuators. Swanson E A and Chinn S R, “Method and Apparatus for Performing Optical Frequency Domain Reflectometry” U.S. Pat. No. 6,160,826 (issued Dec. 12, 2000), Choma M A, Sarunic M V, Yang C, and Izatt J, “Sensitivity advantage of swept source and Fourier domain optical coherence tomography,” Opt. Express, Vol. 11, 2183-2189 (2003).
Instead of wasting available source power by interrogating the sample point-by-point, FD-OCT collects information from multiple depths simultaneously and discriminates reflections from different depths according to the optical frequencies of the signals they generate. FD-OCT systems based on swept-frequency light sources have attracted the most attention for medical applications that require subsurface imaging in highly scattering tissues.
The feasibility of swept-source OCT (SS-OCT) has been demonstrated in several academic research studies. Chinn S R, Swanson E A, and Fujimoto J G, “Optical coherence tomography using a frequency-tunable optical source,” Opt. Lett. Vol. 22, 340-342 (1997); Yun S H, Tearney G J, Bouma B E, Park B H, de Boer J F, “High-speed spectral domain optical coherence tomography at 1.3 μm wavelength,” Optics Express, Vol. 11, pp. 3598-3604 (2003); Choma M A, Hsu K, and Izatt J, “Swept source optical coherence tomography using an all-fiber 1300 nm ring laser source,” J. Biomed. Optics, Vol. 10, p. 044009 (2005); Huber R. Wojtkowski, Taira K, Fujimoto J G, and Hsu K. “Amplified, frequency-swept lasers for frequency domain reflectometry and OCT imaging: design and scaling principles,” Opt. Express, Vol. 13, 3513-3528 (2005). Various SS-OCT systems have been reported including systems based on Fourier Domain Mode Locked (FDML) lasers, surface emitting cavity lasers, short cavity lasers, long cavity lasers, and tunable lasers that include a tunable filter.
Some of the implementations disclosed to date suffer from drawbacks that have discourage widespread commercialization of SS-OCT. For example, certain implementations make real-time data acquisition and display difficult, because they employ data acquisition schemes that require post-acquisition re-sampling or interpolation of recorded data before Fourier transformation. In addition, the relatively short coherence length and tendency for mode-hopping of short-cavity lasers reduce signal-to-noise and image resolution at optical scan depths exceeding 2-3 mm. Many medical applications, including coronary artery imaging, require an optical scan depth that exceeds 5 mm. Some OCT system and method implementations designed to address some of the problems outlined above relating to SS-OCT in context of tunable light sources are recited in Schmitt “Method and Apparatus for Swept-Source Optical Coherence Tomography” U.S. Pat. No. 7,916,387 (issued Mar. 29, 2011) and Schmitt “Method and Apparatus for Swept-Source Optical Coherence Tomography” U.S. Pat. No. 8,325,419 (issued Dec. 12, 2012).
Other approaches relating to SS-OCT and the light sources used with such systems have been proposed. For example, SS-OCT systems using a vertical-cavity surface-emitting laser (VCSEL) laser are described in Jayaraman “System for Swept Source Optical Coherence Tomography” U.S. Pat. No. 7,468,997 (issued Dec. 23, 2008) and Chong “Swept Source Type Optical Coherent Tomography System” U.S. Pat. No. 7,701,588 (issued Apr. 20, 2010).
In general, for SS-OCT systems, it is often the case that the swept light sources that are used exhibit nonlinear sweep patterns. As a result, the optical frequency of light generated by the source does not change linearly over each sweep period. These light sources may also suffer from asymmetry in the forward and backward scans. Further, the nonlinearity and asymmetry problems tend to vary from light source to light source, which makes it difficult, if not impossible, to apply the same corrective action to every unit. A need therefore exists to address such problems. In part, the embodiments described herein address these problems and others relating to certain light sources.